The fabrication of a semiconductor device often requires that a suitable workpiece (e.g. a silicon wafer) be subjected to a sequence of discrete process operations. Many of these processes are very sensitive to the process conditions and are preferably carried out within individual process chambers or work cells, often referred to as process tools, within which very specific conditions are established. Modern semiconductor processing equipment typically utilizes robotic transfer mechanisms to move silicon wafers into and out of these work cells.
The ability to establish and maintain precise conditions within a work cell accurately and reproducibly is needed for the successful production of some of the state-of-the-art silicon devices. In order to achieve the high device yields necessary for commercial success, the conditions within a process chamber are continuously monitored and controlled through the use of sensors designed to measure specific physical parameters. Typically, these control sensors are built into the process tool and measure the parameter of interest (e.g. pressure) at a specific location within the work cell.
As larger work pieces are adopted (e.g. 300 mm diameter silicon wafers), and as the design feature sizes decrease (e.g., 0.13 um transistor gate widths), it becomes important to have each point on the surface of the workpiece processed under optimum process conditions. Measurement of a parameter (e.g. temperature) at an arbitrarily selected point within the work cell may not be adequate to achieve and maintain optimal device yields and performance characteristics. A new type of sensor has been developed to address the need for monitoring process conditions at the work piece surface: U.S. Pat. No. 6,542,835 and U.S. Pat. No. 6,691,068 describe such a sensor system.
Typically, CMP processing is accomplished by pressing the front side (device side) of the semiconductor wafer against a compliant pad. Usually, a liquid solution is introduced between the pad and the wafer. This solution typically contains etching materials and abrasive particles. In some CMP systems, the abrasive particles are preloaded onto and/or into the surface of the compliant pad. By moving the wafer with respect to the pad, material is removed from the surface of the wafer by a combination of chemical etching and mechanical abrasion. Careful control of physical parameters such as contact pressure, slurry composition, surface velocity, pad compliance, etc. results in protrusions on the wafer surface (high spots) being removed at a greater rate than the bulk of the wafer surface. This selective removal of material from the high spots results in the wafer being planarized or flattened. This planarization process is useful in eliminating the uneven surface topology caused by the repeated deposition and patterning (photolithography) steps required to fabricate an integrated circuit.
A second application of CMP processing is in the production of conductive lines or traces via the damascene process. In this process, trenches are etched into an insulator material deposited on the surface of semiconductor wafers. A layer of a conductive material (typically copper) is then deposited or plated onto the wafer surface so as to completely fill the trenches. A CMP process is then used to polish or remove the deposited material back to the original insulator surface, leaving the conductive material filling the trenches.
The quality of the CMP process in terms of removal rates, uniformity, selectivity, etc., is strongly affected by a number of the process variables; the pressure or force with which the wafer or other workpiece is pressed against the pad during the process being a critical factor. Consequently, there is a need for accurate knowledge of the localized pressure distributions (spatial mapping) during actual process conditions. Furthermore, there is a need for methods and apparatus for measuring the evolution of the pressure distributions over time (trajectory); this would provide a valuable tool for optimizing and maintaining CMP processes and process tools.